Implants generally serve as replacements for diseased or lost human or animal anatomical structures, such as teeth, joints, extremities etc. Preferably such implants should knit with the bone in the organism, to form a stable joint that is able to withstand loading long-term. Both titanium implants and ceramic implants are already available. Titanium implants are now well established in medicine, dentistry and veterinary medicine with over 30 years' experience of their use, whereas ceramic implants have only recently begun to be used in implantology. Owing to their excellent biocompatibility, bioinertness, corrosion resistance and their good physical properties, they have become well established in dentistry, mainly through use as implants, but they integrate with bone only poorly, or not at all.
Advantages of titanium are that it has very good osseointegration, i.e. it knits with bone, and it is not allergenic. The high affinity of titanium for oxygen leads to formation of a titanium oxide layer on the titanium implant surface, which leads to the advantageous properties. Bone knits with the titanium oxide layer. In order to maximize the contact surface between implant and bone as far as technically possible, the surface of the titanium implant is roughened. In this way osseointegration can be further improved. Today titanium is used for example for dental implants or in hip joints for titanium cups, which receive a ceramic insert, whereas in orthodontics, among other things anchoring implants made of titanium are used. The use of titanium in restorative dentistry became possible through further advances in casting technology and through the use of CAD/CAM and spark erosion techniques for making individual parts.
However, titanium has the following significant drawbacks, especially for dental implantology:
It has a dark, almost black color and, if it polished to a high gloss, a silvery color, so that the aesthetic appearance leaves much to be desired in the cervical zone of the tooth. Moreover, in dentistry, titanium implants cannot be cleaned, at the point where they emerge from the gum, with ultrasonic tips made of metal, as the material becomes scratched and roughness develops, which promotes increased dental plaque formation. Cleaning therefore requires special plastic tips.
Oxide ceramic (zirconium oxide ceramic, alumina, zirconia-alumina mixtures, etc.) is an extremely hard, smooth, biologically inert material, which is absolutely resistant to corrosion (acid, salts, body fluids). Moreover, owing to its hardness it is extremely abrasion-resistant, i.e. the surface can only be modified using diamond tools. Furthermore, the white color of the material offers—at least for dental implants—excellent aesthetic advantages in dentistry. These properties are already utilized in medicine, e.g. as stents for vessels in cardiology with a surface of ceramic so that there is no build-up of deposits of body cells. The aforementioned advantages are a disadvantage for the ceramic dental implants used in dentistry. Because the material is biologically inert, there is no or only insufficient osseointegration of the implant.
In order to combine the advantages of both materials, oxide ceramics and titanium, and eliminate the respective disadvantages as far as possible, two approaches have been adopted in recent times: implants made of a titanium body with a (partial) ceramic coating (facing) and implants made of a ceramic body with a titanium or titanium oxide coating. In the first approach, those regions of the titanium body that are not in contact with bone after implantation are provided with a ceramic coating. In the second approach, the regions of the ceramic body that are in contact with bone after implantation are coated with titanium or titanium oxide, so that better osseointegration can take place there. The regions of the implant that are not in contact with bone after implantation are left uncoated.
Owing to the material-specific properties of titanium, namely its low coefficient of thermal expansion, the extreme affinity of titanium for air and oxygen and the crystal lattice change at 882° C., the formerly usual metal-ceramic composite systems (metal main body with ceramic surface, facing ceramics) cannot be used, as it is not possible for a ceramic to be “faced” with metal.
Through reaction with ceramic constituents, an oxidative reaction layer already forms on the surface of the titanium body at temperatures of 750-800° C. At temperatures of almost 1000° C., such as are reached in the production of conventional ceramics, there would be extreme strengthening of the oxide layers and therefore the bond to the ceramic coating would be weakened. Moreover, owing to the crystal lattice change, stresses could be a problem, and could also have an effect of weakening the bond. Compared to other dental alloys, titanium has a particularly low coefficient of thermal expansion. The coefficients of thermal expansion of ceramic and metal must, however, be matched to one another, to prevent cracking and spalling of the ceramic, such as would occur on facing titanium with conventional ceramics. As is known by a person skilled in the art, metals expand with heat, whereas ceramics undergo shrinkage during sintering.
For a long time it was not possible to achieve satisfactory values of adhesion strength of titanium-ceramic systems. The lower adhesive bond between titanium and ceramic can be attributed both to the necessary adjustment of the coefficients of thermal expansion, and to the high affinity of titanium for oxygen, so that during firing of the ceramic, there is pronounced growth of the oxide layer. The brittleness of the oxide layer is regarded as the primary cause of the lower bonding values.
For this reason, special binders (adhesion promoters) were developed, which owing to their reducing properties should prevent the oxidation of titanium during firing of the ceramic (M. Kononen and J. Kivilahti, Bonding of low-fusing dental porcelain to commercially pure titanium, J Biomed Mater Res 1994, Vol. 28, No. 9, pages 1027-35; U. Tesch, K. Pässler and E. Mann, Investigations of the titanium-ceramic composite, Dent Lab, 1993, Vol. 41, p. 71-74). In order to compensate the high oxidation tendency of titanium and thus increase the values of adhesion strength of titanium-ceramic systems, special binders were developed, which loosen and envelop oxides present on the titanium surface and, with their glass-like nature, seal the surface against further oxidation (J. Tinschert, R. Marx and R. Gussone, Structure of ceramics for titanium facing, Dtsch Zahnärztl Z, 1995, Vol. 50, p. 31-4). Studies showed, however, that this procedure only led partially to the desired success. Gilbert et al. reported on an improvement of the adhesive bond (J. L. Gilbert, D. A. Covey and E. P. Lautenschlager, Bond characteristics of porcelain fused to milled titanium, Dent Mater, 1994, Vol. 10, No. 2, p. 134-140). However, Hung et al. could not find any significant improvement from using a binder (C. C. Hung, M. Okazaki and J. Takahashi, Effect of Bonding Agent on Strength of Pure Titanium-Porcelain System, J Dent Res, 1997, Vol. 76, p. 60).
A disadvantage of using binders is that another ceramic firing is required which, along with the increased time required, in particular causes additional thermal loading of the titanium. Aesthetic disadvantages caused by the binder also cannot be ruled out.
With the objective of decreasing the oxidation of titanium during firing, tests were undertaken for firing the ceramic under a protective gas atmosphere (J. Geis-Gerstorfer; Ch. Schille and P. Klein, Lower oxidation tendency under protective gas atmosphere, Dent Lab, 1994, Vol. 42, p. 1235-1236), but with only slight success, as mainly the ceramic constituents are made responsible as main supplier of oxygen for oxidation of the titanium (M. Kononen and J. Kivilahti, Fusing of dental ceramics titanium, J Dent Res, 2001, Vol. 80, No. 3, p. 848-854).
Another approach for increasing the strength of adhesion in a titanium-ceramic system is described in DE 10 2004 041 687 A1, according to which a layer of zirconium oxide is applied on a body of pure titanium by a CVD, PVD or plasma-immersion ion implantation and deposition technique, the ceramic for facing the titanium being burnt on without a binder. In this case the zirconium layer serves as adhesion promoter between the titanium body and the applied ceramic layer.
More recent approaches are based on coating a ceramic body with titanium, as it is known that titanium-coated ceramics show very good results with respect to osseointegration. WO 03/045268 A1 discloses, for example, a one-part tooth implant of a ceramic main body with a titanium coating.
However, it is also known that the strength of adhesion between the titanium coating and the ceramic also poses problems, as is known from US 2001/0036530 A1. US 2001/0036530 A1 describes an implant made of a composite material of a zirconium oxide ceramic with a first coating of titanium, a second coating also of titanium and optionally a third coating of hydroxyapatite. In this case, for better anchoring of the first coating and the associated desired better strength of adhesion, titanium ions are implanted in the ceramic by ion implantation. This can improve the strength of adhesion by 20% relative to known ceramic-titanium composite systems. The titanium-ceramic composite systems disclosed do not, however, have satisfactory properties. During investigation of the strength of adhesion, admittedly no cracking or spalling was observed, but the strength of adhesion, averaging 67 MPa, was not significantly above the strength of adhesion of 41 MPa achieved in the prior art. A similar approach was disclosed in EP 2 018 879 A1. However, once again satisfactory strength of adhesion could not be achieved. Thus, in the end, the effect that on loosening the layer, “blank” ceramic makes its appearance, could not be prevented. This effect is not only, but primarily unacceptable in implantology, as material failure has catastrophic consequences, because implants should remain in the body fault-free for decades and optimally for a lifetime.
There are applications, not only but primarily for implants, which require a very high strength of adhesion of the layer. Such applications are not only dental applications, but also other medical uses, such as bipolar prostheses (hemi-endoprostheses) for treating femoral neck fractures. The frequently used dual head prostheses consist of a head, a stem and a socket, consisting e.g. of polyethylene. This leads to the problem that the high mechanical loading causes wear of the polyethylene socket. This wear can lead to loss of sliding properties of the joint. Mainly the abrasion products lead to aseptic bone necrosis. This leads to technical failure of the dual head prosthesis and to consequent damage in the healthy tissue. The above remarks regarding the consequences of the abrasion products also apply to metal-metal pairs and metal-plastic pairs in orthopedic joint prostheses.
There is thus a need for materials for implants that fulfill all requirements in the most varied of applications for implants both from the chemical and the mechanical standpoint. Furthermore, they must have the capacity for osseointegration. There is in addition a need for a method with which these materials can be produced easily and economically in sufficient amounts.